The dendritic trees of the molecular layer interneurons (stellate and basket cells) are oriented in a manner similar to that of the Purkinje cell dendritic tree, although they are much less extensive. The axons of stellate and basket cells run transversely across the folium and form synapses with Purkinje cells. Stellate and basket cells synapse onto Purkinje cell dendrites. In addition, basket cells make synapses on the Purkinje cell soma and form a basket-like structure around the base of the soma, which gives the basket cell its name.
Granule cells are small neurons with four to five short unbranched dendrites, each ending in a claw-like expansion that synapses with a mossy fiber rosette and with terminals from Golgi cell axons in a complex arrangement known as a glomerulus. The axons of granule cells ascend through the Purkinje cell layer to the molecular layer, where they bifurcate and form parallel fibers. The parallel fibers run parallel to the cerebellar surface along the longitudinal axis of the folium (perpendicular to the planes of the Purkinje, stellate, and basket cell dendritic trees) and form excitatory synapses with the dendrites of the Purkinje, Golgi, stellate, and basket cells.
The orthogonal relationship between the parallel fibers and the dendritic trees of the Purkinje cells and molecular layer interneurons (basket and stellate cells) has significant functional consequences. This arrangement allows maximal convergence and divergence to occur. A single parallel fiber, which can be up to 6 mm long, passes through more than 100 Purkinje cell dendritic trees (and also interneuron dendrites); however, it has the chance to make only one or two synapses with any particular cell because it crosses through the short dimension of the dendritic tree. Conversely, a given Purkinje cell receives synapses from on the order of 100,000 parallel fibers. Thus a beam of parallel fibers can be excited experimentally, which excites a row of Purkinje cells and interneurons that are in line with this beam (
Fig. 9.22 ). In addition, because the axons of the interneurons run perpendicular to the parallel fibers, this beam of excitation is flanked by inhibition. Although this classic electrophysiological experiment clearly demonstrates the functional connectivity of the cerebellar cortex, whether such beams of excitation occur normally remains a controversial question.
The Golgi cells are inhibitory interneurons in the granule cell layer. The geometric organization of their axonal and dendritic arbors is an exception to the orthogonal and planar organization of the cortex in that their dendrites and axons carve out approximately conical territories: like two cones, tip to tip, in which the soma is at the point where the two cone tips meet. The dendritic tree forms the upper cone, which often extends into the molecular layer, and the axon forms the lower one. Golgi cells are excited by mossy and olivocerebellar fibers and by granule cell axons (parallel fibers) and inhibited by basket, stellate, and Purkinje cell axon collaterals. They in turn inhibit granule cells. Thus they participate both in feedback (when excited by parallel fibers) and in feedforward (when excited by mossy fibers) inhibitory loops that control activity in the mossy fiber–parallel fiber pathway to the Purkinje cell.
Lugaro cells have fusiform somata from which emerge two relatively unbranched dendrites, one from each side, that run along the transverse axis of the folium for several hundred microns, usually just under the Purkinje cell layer. Purkinje cell axon collaterals provide the main input to these neurons, and granule cell axons add minor input. The axon terminates mainly in the molecular layer on basket, stellate, and possibly Purkinje cells. Thus these cells appear to sample the activity of Purkinje cells and provide both positive-feedback signals (they inhibit the interneurons that inhibit Purkinje cells) and negative-feedback signals (they directly inhibit the Purkinje cell).